1. Field of the Invention
The present invention relates to a deformable mirror for an aberration corrector which is used for light pick-up for reproducing data from any of optical data recording media such as optical disks having thickness variations and adapted to correct spherical aberration of the mirror for correctly focusing light rays on a data recorded surface of an optical disk. Further, the invention relates to an optical data reproducing apparatus using the deformable mirror.
2. Description of the Related Art
Digital versatile disks (DVDs) have been receiving attentions as a new type of optical data recording media. Since the DVDs are designed to record data at a higher density than the conventional CDs, various modifications have been made to the DVDs themselves and to optical systems of optical data reproducing apparatuses.
When data are to be reproduced from a CD by means of a data reproducing apparatus designed for the DVDs, defocusing occurs due to a difference in thickness between the CD and the DVD (DVD: 0.6 mm, CD: 1.2 mm), making it difficult to obtain high quality data reproducing signals. To cope with this problem, an defocusing correction function should be incorporated in the optical system of the data reproducing apparatus.
Typical aberration correctors for correcting the defocusing are disclosed in Japanese Unexamined Patent Publication No. Hei 5(1993)-151591 and the like. One exemplary aberration corrector is shown in FIG. 21. The aberration corrector 630 has a pair of complementary triangular blocks 632 and 634. The block 634 is attached to a voice coil motor 636 and slidable on a surface of the block 632. The total thickness of the blocks through which light beam 602 passes can be controlled by moving the block 634 relative to the block 632, thereby controlling aberration to be imparted to the light beam 602. In FIG. 21, there are also shown a condenser lens 610 and a disk 612 from which data are reproduced.
Another exemplary aberration corrector is shown in FIG. 22. The aberration corrector 720 has a polarization beam splitter 722, a quarter-wave plate 724, and an electrically controllable deformable mirror 726. Light beam 702 incident to the polarization beam splitter 722 passes through the quarter-wave plate 724, and is subjected to spherical aberration control by the deformable mirror 726. The light beam again passes through the quarter-wave plate 724, and then is guided in a direction perpendicular to the incident direction thereof by the polarization beam splitter 722.
More specifically, the electrically controllable deformable mirror 726 includes, as shown in FIG. 23, a deformable plate 801 having a mirror face 800 on its surface, piezoelectric actuators 802 for pressing the deformable plate 801 from the back side thereof at several points, and a base plate 803 on which the deformable plate 801 and the piezoelectric actuators 802 are fixed.
By variably controlling voltages to be applied to the respective piezoelectric actuators 802, the deformable plate 801 is deformed by a desired degree, so that the mirror face 800 on the deformable plate 801 can be deformed into a desired configuration. Thus, the spherical aberration to be imparted to the light beam 702 can be controlled when the light beam 702 is reflected on the mirror face 800.
However, these aberration correctors have the following drawbacks. The aberration corrector 630 shown in FIG. 21 employs the voice coil motor 636 for the aberration control and, therefore, is not advantageous in the size reduction and energy saving of an optical data reproducing apparatus.
The aberration corrector 720 shown in FIGS. 22 and 23 is constructed such that the piezoelectric actuators 802 press the deformation plate 801 from the back side thereof at the several points. Therefore, the mirror face 800 is not necessarily deformed in a desired configuration because of environmental influences such as vibrations and a temperature change. Since the light beam 702 to be used in an optical data reproducing apparatus typically has a diameter of about 4 mm, a multiplicity of piezoelectric actuators 802 should be provided within a 4 mm diameter area to precisely deform the deformable plate 801 in the desired configuration. This complicates the construction and assembly of the aberration corrector. In addition, the size of the deformable mirror 726 is increased because the multiplicity of the piezoelectric actuators 802 should be accommodated therein and connected to interconnection lines. Therefore, the aberration corrector 720 is not advantageous in the size reduction of a light pick-up device.